The invention relates generally to fluid power systems. More particularly, the invention relates to a fluid power system which employs hydraulic pressure to provide load bearing force and extension control.
Fluid power systems are employed in a wide variety of small-scale and large-scale industrial applications. For example, fluid power systems are used to generate large compressive forces in plastic molding, multiply input force in braking systems, adjust elevation in jacks, lift weight in cranes, and provide control actuators in steering systems. In the offshore oil industry, hydraulic systems are employed in applications such as subsea template leveling, riser tensioning, and equipment jacking. Many of these applications require long-term service, exposure to corrosive environments, or even immersion in seawater.
Fluid power systems typically employ hydraulic cylinders to provide load bearing force and elevation control. As illustrated in FIG. 1A, a hydraulic cylinder 100 conventionally comprise a piston 102 and piston rod 104 disposed within a cylinder 106. A sliding O-ring seal 110 and a rod wiper 112 are typically positioned about an aperture 114 in the cylinder 106. The sliding seal 110 and wiper 112 act to seal fluid in the cavity 116 of the cylinder 106 while permitting extension and retraction of piston rod 104 with respect to cylinder 106. In operation, the piston 102 also comprises one or more sliding seals forming to isolate fluid pressure on either side of the piston 102. Fluid 118 is generally drawn from a reservoir 120 through a pump 122. A control assembly 124 directs fluid flow. As illustrated by reference to FIGS. 1B-C, differential pressure (P1 greater than P2) provided by the pump 122 acts on the surface areas of the piston 102 to induce a force which extends (F1) or retracts (F2) the piston rod 104. The magnitude of extension or retraction force is generally described by the relationship of F=PA, where: (F) is the extension or retraction force, (P) is the differential pressure (P1-P2) and (A) is the surface area of the piston upon which the pressurized fluid acts.
Conventional hydraulics are widely employed. Several aspects of conventional hydraulic cylinders, however, inherently limit their application. The first limiting factor is the difficulty in forming a seal which can contain the high-pressure fluid within the hydraulic cylinder while simultaneously permitting relative movement of the cylinders. Sliding seals are prone to leakage, wear, and failure under high pressure and generally require periodic monitoring and replacement. A second limiting factor relates to the size of the hydraulic cylinder. The maximum size of the seal is generally limited due to difficulties in the manufacturing process. This limited seal size results in smaller-diameter hydraulic cylinders which require higher working fluid pressures to generate loads.
In addition to the two limiting factors mentioned above, conventional hydraulic cylinders require strict tolerances on cylinder machining and O-ring fabrication. The fluid employed in hydraulic cylinders is generally an oil derivative. This type of fluid is selected to prevent the leakage or seal corrosion that can occur with other fluids such as seawater or fresh water. The hydraulic fluid can, however, be an environmental contaminant and can be expensive where large quantities are required.
It is, therefore, desirable to provide a fluid power system which does not employ sliding seals or moving parts. It is further desirable to provide a seal with less strict manufacturing tolerances, reduced maintenance, and lower failure rates. It is also desirable to provide a system that can employ inexpensive and environmentally friendly fluids, such as fresh or seawater. It is still further desirable to have a system that can be fabricated in large diameters to provide large extension forces with relatively lower working fluid pressures.
The principle of nested cylinders having relative displacements is employed in other technical fields, such as bonded rubber shear springs. The offshore oil industry employs bonded rubber shears springs for use as marine shock cells in various applications. The general function of a marine shock cell is to absorb impact loads, such as those induced during the docking operations of a ship to an offshore oil structure. As illustrated in FIGS. 2A-B, a conventional marine shock cell 200 comprises an inner cylinder 202 and a larger diameter outer cylinder 204. An elastomer annulus 206 spans the gap between the inner 202 and outer 204 cylinders. The elastomer annulus 206 is bonded to the outer surface 208 of the inner cylinder 202 and the inner surface 210 of the outer cylinder 204 during the molding process. The application of a force (F) to the shock cell 200 induces deflection (xcex4X). A designer arranges such variables as cylinder diameter (D), gap length (L), thickness of the elastomer annulus (T), and elastomer mixture in order to produce a desirable reaction force versus deflection characteristic for the shock cell 200. A generalized reaction force versus deflection characteristic for a shock cell is illustrated in FIG. 2C. Generally, the elastomer annulus 206 of the shock cell 200 resists deflection with an increasing force (F) as deflection (xcex4X) increases. The area under the curve (A) corresponds to the quantity of impulse energy, or shock, absorbed by the shock cell through full deflection. Upon removal of the external force (F), the shock cell will return to an undeflected condition.
A shock cell provides elongation without the use of a sliding seal. Manufacturing tolerances are generally low. Marine shock cells function for decades without maintenance or failure. Shock cells can be manufactured in extremely large diameters. The function of a shock cell or other shear spring, however, is generally the inverse of the function of a fluid power system. A shock cell is a reactive device absorbing external energy input. A fluid power system actuates external energy input to provide power output. The elastomer annulus of a shock cell is designed to impede relative movement between cylinders and is not optimized to form a fluid tight seal. The sliding seal of a fluid power system is designed to enable relative movement between piston rod and cylinder.
In general, in one aspect, the invention relates to a load bearing device which comprises an extendable, close-ended container having a first cylinder and a second cylinder, the first cylinder coaxially disposed about the second cylinder and having a diameter which is larger than a diameter of the second cylinder. A first elastomer annulus having an outer circumference bonded to an inner surface of the first cylinder, and an inner circumference bonded to an outer surface of the second cylinder, and first means for pumping a first fluid into and out of the container, wherein the bonds between the elastomer annulus and the first and second cylinders form fluid-tight seal for the container.
In general, in another aspect, the invention relates to a load bearing device which comprises at least one expansion segment comprising a first cylinder coaxially disposed about a smaller diameter second cylinder. An elastomer annulus has an inner circumference bonded to an outer surface of the second cylinder and an outer circumference bonded to an inner surface of the first cylinder. End caps enclosing a cavity is formed by the first and second cylinders and the elastomer annulus, and pumping means is included for adjusting a volume of fluid in the cavity, wherein changes in the fluid volume in the cavity induce relative displacements between the first and second cylinders.
In general, in another aspect, the invention relates to a method of bearing a load. The method comprises providing an extendable, close-ended container having a first cylinder and a second cylinder, the first cylinder coaxially disposed about the second cylinder and having a diameter which is larger than a diameter of the second cylinder, the first and second cylinder connected together by an elastomer annulus so as to form a fluid-tight seal between the elastomer annulus and the first and second cylinders, and inducing relative movement between the first and second cylinder by adjusting a volume of fluid within the container.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.